The present invention generally relates to the production test requirements for testing programmable impedance drivers such as the BZIO buffers contained in RapidChip® and ASIC devices, and more specifically relates to a four point measurement technique.
The problem faced in the manufacturing test environment is that it is a non-ideal situation with respect to the contact resistance that occurs at multiple points between the tester's pin electronics and the actual device-under-test (DUT). These contact resistances are difficult to control at best, and cannot be completely eliminated. As such they contribute an error component to any resistance measurements that are to be made on the actual DUT. These errors in the measurements result in failing tests during the manufacturing test flow causing product yield issues.
FIG. 1 represents the typical circuit for an n-channel device which is to be measured on the DUT, including the error components which are an inherent part of the measurement process. A programmable impedance driver typically consists of several transistor ‘legs’ which can be selectively turned on to provide varying drive strengths. Regardless of the combination of those various transistor legs, the final effective drive strength can be viewed as a lump-sum resistance of the transistors in their ‘ON’ state. This is the value RTransistor shown in FIG. 1. The labels ‘Ground’ and ‘Signal Pad’ refer to the contact points (identified with reference numerals 10 and 12 in FIG. 1) between the automatic test equipment (ATE) pin electronics and the DUT, while ‘RGround’ and ‘RSignalpad’ refer to the contact resistances associated with those connections. These contact resistances are what contribute to the error components of the test measurement, resulting in the inability to accurately test the RTransistor value. The standard approach to measuring the on-resistance of a transistor or transistor network on the ATE is to inject a current at ‘Signal Pad’ (i.e., at point 12 in FIG. 1) and measure the resultant voltage at the pad. The transistor on-resistance is then calculated using Ohm's law, or R=V/I. The problem is that the calculated on-resistance includes the unknown error components associated with ‘RGround’ and ‘RSignalPad’.
The only existing solutions to the aforementioned problems involve the relaxation of test limits for the DUT, or the elimination of the test altogether. While this can address the manufacturing test problem, it provides the risk of shipping product out which is out of specification. Alternatively, the testing can be performed within the specified test limits, and the manufacturer is forced to accept any associated yield losses during the manufacturing test process.